The present invention relates to a hydraulic clutch actuation system having a hydraulic clutch control device which includes a first clutch master cylinder that communicates via a first pressure-medium line with a first clutch slave cylinder used for actuating a first clutch device, and a second clutch master cylinder that communicates via a second pressure-medium line with a second clutch slave cylinder used for actuating a second clutch device, the pressure prevailing in the first and/or in the second pressure-medium line being relievable via at least one emergency release valve device to permit disengagement of at least one of the clutch devices in an emergency situation.
The clutch devices are preferably friction clutches. The hydraulic clutch actuation system is also described as a hydraulic clutch-release system, which includes manually or automatically actuated master cylinders and slave cylinders that disengage the clutches. In the case of so-called external pressure-operated clutches, the clutch is disengaged in the force-free state of the clutch, and the frictional engagement for connecting an internal combustion engine and a transmission is accomplished by actuation of the slave cylinder and holding the same under pressure via the master cylinder. For twin clutches, in particular, in which a shared input part branches off to two output parts, each having one transmission input shaft, this type of clutch actuation is advantageous. Should both or one of the clutch control devices, also described as clutch release devices, fail, for example due to loss of pressurizing medium, then these clutches remain disengaged and do not lock up the transmission, as do clutches, for example, which are normally engaged in the force-free state. In addition to their use in manually operated transmissions, friction clutches of the mentioned type are typically used in automated transmissions, such as in automated manual transmissions (AMT) or twin-clutch transmissions (TCT), which are generally controlled by a control unit, the clutch operation, as well as engagement and disengagement of the gears being automated. In the event that a control unit of this kind fails, depending on the control strategy being used, the situation can arise that the clutch position of one clutch, respectively in a twin-clutch transmission, of both clutches, freezes, with the result that the operation of the internal combustion engine can no longer be controlled independently of the transmission output speed and, thus, of the wheel speed, i.e., in the case of a vehicle at standstill, it is no longer possible to operate the internal combustion engine in order to sustain auxiliary systems. For example, in the event of failure of the transmission control, the pressure prevailing in the first and/or in the second pressure-medium line can be relieved by an emergency release valve device in order to disconnect a clutch that no longer disengages.